I found 9999 bones to truly be a lot more of a pain than insane or Hero of Shattrath due to the fact that it's little more than just one-shotting mobs over and over.

Most annoying, solo-wise, must've been Disruptron (horde, red floor, red lasers) post-levitate fix.
I'm only missing Show Me Your Moves! for Glory of the Pandaria Raider and fucking hell, is it a brickwall to pug.

Mine would have to be [I can't hear you over the sound of how awesome I am] , same group had be running it for a few weeks and something always went wrong, the stress of leading them was insatiable, however the week we done it, the nerd screams were huge!

-Herald of the Titans
Weeks of leveling up an alt, months to gear him up, often being laughed at by others for my efforts. In the end i spent various thousands of gold to get into a sure kill run. Was worth it.

Get strongest Gear on max Level, use Flask Food Potion and do 5 Seconds burst. Should I feel good or bad?

Salty on a high-pop (usually hovering around Full or Locked) PvP realm pre-CRZ. Every sunday, you'd see every dirty trick in the book being pulled out, most of which would warrant temp bans at the least, but as the answer times on tickets were usually in the days region, there was no point filing them.

...Good question really. I never did many PVE achivements when it was bleeding-content, new and fresh, so maybe if I was abit more hard-core that would have been the case.

One reason for WoW decline I often hear is that its an old game and naturally people get bored with it. But TV is much older and people still watch that. Surely if a thing you once enjoyed keept staying fun, you would still use it?

I always figured that the rank 1 Gladiator title would be the most difficult, anyone here gotten it?

Most people would rather die than think, and most people do. -Bertrand Russell
There is not a flag large enough to cover the shame of killing innocent people. -Howard Zinn
Reality is that which, when you stop believing in it, doesn't go away. -Philip K. DickThe nationalist not only does not disapprove of atrocities committed by his own side, but he has a remarkable capacity for not even hearing about them. -George Orwell

Alterac Valley All-Star solo was kind of hard always something going wrong. Most of the time it was defending a tower but it happened that I never got the chance to capture or defend a graveyard, it have probably been achievements that have been harder for me but this is what I can think of right now.

Battlemaster - which incidentally includes a lot of the battleground-related achievements mentioned already in this thread. I did it pre-nerf of "We had it all along *cough*".

It's also the one I'm most proud of - not because it's flashy or draws a lot of 'Damn, ur awesum!'-remarks and admiration - but because I know how grueling it was and how satisfying it was for me to get it. Most of the achievements was done solo and even though I did inspire some to join me at some point, they fell off one by one, because they lost hope.

Brawler's Guild is actually a challenge in appropriate gear, I got R10 as a warrior in ~525-530 ilevel, I think it was server first warrior. (according to Guild Ox)

Masters of battlegrounds is also hard for me, it's just so boring grinding the same instance 120-150 times, PvP is just so boring, classes never change except maybe small abilities, it's been the same way for almost 10 years.

I know this is going to be common response but as previous poster said

Insane if it was Pre- Nerf. Pre-Cata
Shen'dralar and DMF specifically

Going to agree strongly here. Prior to the nerf this was a really time consuming achievement that required patience, time and being able to play the AH reasonably well.

Since Cataclysm, and especially since the introduction of DMF, it has become leaps and bounds easier. This isn't bad in and of itself, but we still haven't been promised the supposed new rep grind that would replace it as being absolutely hard as nails to get.

The rare killing achievements, Bloody rare, Frostbitten and Glorious!, with Glorious! being the easiest one.
Not hard skillwise, but boring and time consuming on a high pop server. I still have nightmares about Vyragosa.

'Monogamy'? Is that one of the ways people in Boringland bore each other?-Curvy